1. Field of the Invention
The invention is generally related to floating structures offshore for oil and gas production and, more particularly, to a riser keel joint assembly for such structures.
2. General Background
All floating systems used by the Oil and Gas Industry to recover hydrocarbons from seafloor sites in offshore waters have risers of some type connecting the well termination at the seafloor to the floating system at the surface. One particular type of riser, the independently supported, top-tensioned riser, extends vertically from the seafloor to the floating system and is directly supported either by buoyancy modules (cans) or other means (e.g., tensioners) that can support the weight of the riser and accommodate the relative movement between that riser and the floating platform when the platform responds to metocean environments. This type of riser has been used by both Spar platforms and Tension Leg Platforms. Where the platform hull is a mono-column-or these risers pass close by the hull structure, some kind of special section of riser is required at the keel of the hull to accommodate the bending loads where the riser leaves the support of the platform and this section also has to accommodate the relative vertical movement between the riser and the hull.
The special riser joint at the keel of the hull and which is addressed by this invention is commonly referred to as the Keel Joint. This section is reinforced to carry the bending loads imposed on the riser by the pitch/heel motions of the hull relative to the riser as well as the bearing and wear loads imposed on the riser by the vertical and lateral motions of the hull relative to the riser.
The functions of a keel joint are straightforward and include:
Reinforcing the bending capacity of the riser by a significant amount so it can have adequate strength and adequate fatigue life (lower stress ranges).
Permitting the riser pipe to curve compliantly as the hull keel moves horizontally relative to the fixed end of the riser at the seafloor.
Bearing on the guides in the hull both to transfer the load to the hull through the keel joint outer surface, instead of through the riser pipe itself, and to incur the wear from friction forces as the riser slides axially against the guides in the hull.
There are several versions of keel joints in the known art.
One type has a larger diameter sleeve, centralized around the riser pipe and attached directly to it with rubber spacers at each end which are vulcanized to both the riser and the sleeve in the annular space. This type of joint supports the riser at the two locations of the rubber and delivers the lateral load from these two locations through the sleeve to the guide locations(s). The rubber provides the flexibility for the riser itself to rotate. In this version, the keel joint is an integral part of the riser string itself.
Another type has the riser in a sleeve similar to the above type but the sleeve is attached to the riser by bolting at each end. For this purpose, the riser is fabricated with machined bumps and flanges at each end both to attach to the sleeve and to the continuing sections of riser at each end. Riser rotation is limited by the flexibility of the sleeve and the riser pipe itself beyond either end of the sleeve resulting in a rather stiff system in bending.
Another type has the riser centralized in a larger diameter pipe called a stem. The stem is suspended directly from the buoyancy module at the top of the riser. The stem performs the same function as the sleeve in the aforementioned example but in this version the riser is not connected to the stem but only centralized within it using a ball type centralizer that allows the riser to pivot and curve relative to the stem.